I decided to replace an ailing NiMh battery pack (3.6 V) with 3 AA alkalines in series (4.5 V) to a small DC motor (hair clippers). Soldered the batteries in proper polarity series and ran the motor for a few minutes. The batteries became so hot they nearly exploded. Any ideas why this might have happened?
Its just the Draw of that Motor was a bit on the Amperage side. Hence causing a good workload, therefore you get Heat. Ever Notice how a Good Flashlights batteries get Warm?
Battery voltages are not constant. The rating you see on the side is a nominal value which is in the range expected for the battery. The 3.6 NiMh battery pack output about might be from 2.8v to 3.6v . Alkalines are rated at 1.5v however if you check a new one, you will see that they actually output 1.6-1.7 when new. If your motor was designed to run in a 3.6 volt NiMh battery pack, the intended voltage for the motor might be in the range of 2.5v to 3.6v. They would want the motor to continue to operate as the pack discharges. When you connected the 4.5v to 5.1v battery pack to the you nearly doubled the voltage and current flow. This allowed the motor to pull a bunch of current from the batteries and heat them up. The heat increased the voltage of the batteries and caused them to put out more current and more heat.
connect in sequence merely lay in to its length by way of connecting the top of the battery A to the backside of battery B, the top of battery B to the backside of battery C, the top of battery C to the backside of battery D. destructive-useful -destructive-useful -destructive-useful -destructive useful here the way it is going to look like in sequence -+ -+ -+ -+ connect in Parallel connect all the top of battery A,B,C,D all jointly connect All backside of battery A,B,C,D all jointly here the way it is going to like in parallel ++++ - - - -
Here’s two possible reasons. 1. The original NiMh battery pack may have been proprietary and had an internal current-limiting resistor where the off-the-shelf cells would not. 2. Let’s use simple numbers and say the motor has a resistance of 10 ohms. At the original 3.6v, it’s drawing 0.36 amps (Voltage / Resistance) and 1.296 watts (Voltage squared / Resistance). At the new 4.5v, it’s now drawing 0.45 amps and 2.025 watts – a 25% increase in current but a 56.25% increase in power. And these calculations do not take into effect if there is a current-limiting resistor in the NiMh pack so there is a possibility the new cells are drawing even more. As much as it may be tempting to try and replace a battery pack in the manner you did, it’s rarely a good idea unless you have basically an exact match replacement. Hope this helps.
It is possible that the battery pack had cells in series to obtain the correct (added) voltage and also series cells in parallel so that all the current did not have to travel through each cell. Sharing the load (parallel cells) would reduce current and heat. Also the three AA alkaline cells in series produced a higher voltage than the battery pack resulting in greater current and the load (hair clippers) may have had too little resistance appearing similar to a short circuit. The current in the load also passes through all of the cells which have internal resistance and can heat as a result of current (I square R losses).
